Inkjet nozzle flushing mechanism

ABSTRACT

A method is disclosed. The method includes receiving print job data, rasterizing the print job data to generate image data for each page of the print job data to be printed, calculating a coverage per unit area for two or more color planes of each page of the image data and calculating a flushing mask to flush nozzles of the inkjet print head.

FIELD OF THE INVENTION

The present invention relates to the field of printing, and inparticular, to flushing the nozzles in an inkjet printer.

BACKGROUND

An ink jet printer is an example of a printing apparatus that ejectsdroplets of ink onto a recording medium such as a sheet of paper, forprinting an image on the recording medium. The ink jet printer includesa head unit having at least one ink jet head provided with an inkcartridge that accommodates the ink. In operation of the head unit, theink is supplied from the ink cartridge to each ink jet head havingejection nozzles, so that a printing operation is performed by ejectionof the ink droplets from selected ejection nozzles.

However, ink jet printers may suffer from a problem of evaporation ofsolvent from the ink causing an increase in the ink viscosity that leadsto nozzle clogging and the inability to fire an ink droplet under normalconditions. A clogged nozzle may not only result in diminished printquality, but may require the expense of replacing the entire print head.To solve this problem, there has been practiced a so-called “flushingoperation” wherein the ink is forcibly discharged from the ejectionnozzles which are open in a nozzle surface of each ink jet head.

Several flushing methods in existence have undesirable effects uponimage quality. One such flushing method involves printing a line acrossthe top or bottom of each page to flush the nozzles. In this method,each nozzle produces multiple drops forming a wide line across the topor bottom of the printed page. The drawback of this approach is that itleaves a large colored line at the bottom of every page and manycustomers do not have the post-processing equipment to remove it.

Another method involves randomly firing drops from all nozzles at aspecified frequency throughout the printing of a job. The drawback tothis approach is that the indiscriminate firing of the nozzles duringthe printing can cause excessive background noise and alter the colorand accuracy of the printed images.

Intelligent flushing methods exist that lessen the problem of theflushes interfering with image quality, but these methods are alsoundesirable because they are computationally intensive. Thus, thethroughput speed of the printers is negatively affected. One such methodinvolves flushing the color ink dispensing nozzles onto points of thepage where black ink will ultimately be printed. By effectively hidingthe color ink droplets under black ink from the job data, the imagequality may be preserved. However, this method requires additional dataprocessing, slowing down the high speed printing process.

Additional problems exist with the current flushing methods. Forinstance, current flushing methods require all nozzles to be flushed atthe same frequency. This results in wasted ink where a user knows thatone color needs to be flushed less frequently than another.

Consequently, what is a needed is a mechanism for flushing the nozzlesof an inkjet print head during printing that preserves the integrity ofthe printed images.

SUMMARY

In one embodiment, a method is disclosed. The method includes receivingprint job data, rasterizing the print job data to generate image datafor each page of the print job data to be printed, calculating acoverage per unit area for two or more color planes of each page of theimage data and calculating a flushing mask to flush nozzles of theinkjet print head.

In further embodiment, a printer is disclosed. The printer includes acontrol unit to receive print job data, rasterize the print job data togenerate image data for each page of the print job data to be printed,calculate a coverage per unit area for two or more color planes of eachpage of the image data and calculate a flushing mask. The printer alsoincludes an inkjet print head having a plurality of ink nozzles flushedaccording to the flushing mask.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention may be understood more fully fromthe detailed description given below and from the accompanying drawingsof various embodiments of the invention. The drawings, however, shouldnot be taken to be limiting, but are for explanation and understandingonly.

FIG. 1 is a block diagram illustrating one embodiment of a print system.

FIG. 2 illustrates one embodiment of a page printed at the print system;

FIG. 3 is a flow diagram for one embodiment of performing a flushingoperation;

FIG. 4 illustrates one embodiment of color coverage area calculations;

FIG. 5 illustrates one embodiment of normalized color functions;

FIG. 6 illustrates one embodiment of re-scaled color functions;

FIG. 7 is a flow diagram for one embodiment of merging a flushingpattern with print job data;

FIG. 8 illustrates embodiments of flushing patterns;

FIGS. 9A and 9B illustrates embodiments of merged flushing patterns withprint job data; and

FIG. 10 illustrates one embodiment for flushing a multi-channel inksystem.

DETAILED DESCRIPTION

An inkjet printer flushing mechanism is described. In the followingdescription, for the purposes of explanation, numerous specific detailsare set forth in order to provide a thorough understanding of thepresent invention. It will be apparent, however, to one skilled in theart that the present invention may be practiced without some of thesespecific details. In other instances, well-known structures and devicesare shown in block diagram form to avoid obscuring the underlyingprinciples of the present invention.

Reference in the specification to “one embodiment” or “an embodiment”means that a particular feature, structure, or characteristic describedin connection with the embodiment is included in at least one embodimentof the invention. The appearances of the phrase “in one embodiment” invarious places in the specification are not necessarily all referring tothe same embodiment.

FIG. 1 illustrates one embodiment of a printing system 100. Printingsystem 100 includes a print application 110, a server 120 and a printer130. Print application 110 makes a request for the printing of adocument. In one embodiment, print application 110 provides a MixedObject Document Content Architecture (MO:DCA) (also called an AdvancedFunction Presentation (AFP)) data stream to print server 120. Althoughin other embodiments, alternative presentation formats (e.g., PortableDocument Format (PDF) data) may be provided by print application 110.

Print server 120 processes pages of output that mix all of the elementsnormally found in presentation documents, e.g., text in typographicfonts, electronic forms, graphics, image, lines, boxes, and bar codes.In one embodiment, print server 120 communicates with a control unit 140within printer 130 to facilitate the interactive dialog between printserver 120 and printer 130. Printer 130 also includes a print head 160.Control unit 140 processes and renders objects received from printserver and provides sheet maps for printing to print head 160. Controlunit 140 includes a rasterizer 150 to prepare pages for printing.

Particularly, rasterizer 150 includes a raster image processor (RIP)that converts text and images into a matrix of pixels (bitmap) that willbe printed on a page. In one embodiment, print head 160 is a fixed,wide-array inkjet print head including one or more nozzles 170 that areimplemented to spray droplets of ink onto a sheet of paper in order toexecute a print job. However, print head 160 may include other types ofink jet print heads, as well as a moving print head design.

As discussed above, nozzles 170 may suffer from an increase in inkviscosity that leads to clogging and the inability to spray ink. Forinstance, referring to FIG. 2, a page 200 is illustrated representing adocument printed at printer 130. As shown in FIG. 2, page 200 includesvarious white spaces that represent areas where no ink is applied. Thus,whenever printer 130 prints a job having a large number of pages similarto page 200, a number of nozzles 170 are seldom used, if used at all.Therefore, a flushing operation is performed at printer 130 in order toforcibly discharge ink from the nozzles.

According to one embodiment, control unit 140 facilitates flushingoperations at nozzles 170 that are inadequately used during the printingof jobs. FIG. 3 is a flow diagram for one embodiment of a processperformed by control unit 140 to facilitate nozzle flushing. Atprocessing block 310, a coverage per unit area of each sheet-side to beprinted is calculated.

At processing block 320, the visual appearance of image data from othercolor planes is accounted for. At processing block 330, a randomflushing mask is calculated in order to achieve particular criteria. Inone embodiment, the criteria includes minimizing an amount of flushingink required, achieving a pleasing human visual system (HVS) mask foreach color plane when flushing density is sufficiently high to bevisible on a printed medium (e.g., paper) and adjusting flushing densitybased on digital counts for the various color planes.

In one embodiment, process of calculating the coverage per unit areasincludes performing the calculation for each color plane. In such anembodiment, coverage for each color plane is defined as an averagedigital count divided by 2.55 for an eight bit image system over adefined/unit area. For the purpose of this description, the unit area is1 inch by 1 inch, with the number of sample points being 16 per 1 squareinch. Thus, the coverage is calculated over a 1 square inch area, wherethe sample point is the center of a 1 square inch area

Because the calculations are performed every ¼ inch, the surface formedbased on the sample points effectively passed through a low pass filterso the coverage function has limited high frequency information tominimize rapid change. FIG. 4 illustrates one embodiment of colorcoverage area calculations with interpolating between sample points.

Subsequently, the coverage functions shown in FIG. 4 are converted toprovide an estimate of a probability function relating when flushing isrequired. In one embodiment, the conversion process includes normalizingeach coverage function in the FIG. 4 plots by dividing the coveragefunction by 100 and inverting the resulting function by subtracting thefunction from 1. FIG. 5 illustrates one embodiment of the results forthe normalized and inverted color functions.

Next, the resulting function is re-scaled to account for the differencebetween the function above and the effect of nozzle clogging not relatedto ink volume ejected. In one embodiment, the scaling includessaturation so that once a certain ink volume is jetted per unit area noadditional jetting is required to prevent clogging. In one embodiment, abasic fundamental assumption for this process is related to actual inkvolume jetted per unit time. Because medium velocity in the processdirection is constant, the process is related to actual ink volumejetted per unit area. FIG. 6 illustrates one embodiment of re-scaledcolor functions. The results shown in FIG. 6 assumes that the functiondescribed by FIG. 5 is cubed to account for nozzle clogging not relatedto actual ink volume jetted per unit area.

The process of calculating a flushing mask includes superimposing aconstant halftone tint over a printed page independent of print job pagedata. In one embodiment, the mask places a pleasing image over the pagesto ensure all ink jet nozzles are adequately used. To achieve theabove-recited criterion of minimizing ink flushing utilization isimplemented by modulating a constant level using a probability function,where flushing is required as a function of page location.

In one embodiment, the probability function is described by the surfacefunction described in FIG. 6. In a further embodiment, the function maybe represented as a multi-dimension LUT or a curve fitted function(e.g., curve fitting techniques). The product of a constant and theprobability function describes the flushing tone density to beimplemented across a sheet-side if the interactions with other colorplanes are neglected. This result is called tone density function.

To account for interaction with other color planes, the tone densityfunction is multiplied by a function (either continuous function orlogic function) accounting for the interaction with other color planes.In one embodiment, the function relates flushing to the other colorplanes where the flushing density should be either increased ordecreased for various reasons.

The resulting function may be referred to as a flushing densitymodulation function. Interaction with other color planes is accountedfor because a cyan, magenta, or black flushing drops in a solid yellowarea is perceived as not a pleasing appearance, and because increasingthe flushing density for allowable areas by takes into account the areasnot available due to other color plane interactions.

The above described flushing mask formation performs random flushingindependent of counting the number of times each individual nozzleejects ink drops. Thus, the flushing pattern is applied in the locationdesired instead of depending on feedback from individual ink jet nozzlesejection count. However, the described embodiment may be computationalexpense despite being highly functional. Accordingly, variousembodiments may be implemented to simplify the procedure in order torealize a practical implementation.

In one such embodiment, the flushing pattern is made to depend onlocation (for visually pleasing characteristics) and does not depend onthe intensity and/or digital count. In this embodiment, the flushingintensity modulation function is factored out of the visually pleasingprocess, (e.g., product of the function and the process for a constantlevel). The visually pleasing overlay can be generated for the case whenthe probability of flushing required equals 1. The visually pleasingoverlay is multiplied times the probability to achieve desiredpersonalized overlay for the sheet-side. Thus the overlay combination ofvisually pleasing and the probability function varies the flushingdensity using drop size where the location for every drop (independentof size) is the same.

In another embodiment, the low pass (filtering) averaging is eliminated.Thus, the calculation depends on a single coverage sample per unit areaconverted to a probability with interpolation between sample points. Analternative embodiment uses one of several visually pleasing masks basedon the probability for the area the mask covers (e.g., with or withoutlow pass filtering). This embodiment allows a possible noticeable stepchange in background if visible.

In yet another embodiment, the computational expense is minimized forcontroller 140 by generating a lookup table LUT relating the planeintensity value to the desired drop size for flushing. The flushingoverlay is generated for the case when all white pages are printed (orequivalently the probability flushing required equals 1). Subsequently,the plane at the pel locations is sampled where the flushing overlaydefines a drop of ink. From the LUT, the digital value in the overlay ischanged to correspond to the correct ink drop size.

An exemplary system implemented to simplify the procedure may includeprinter 130 rasterizing and halftoning the print data, and then adding aflushing pattern to the print data. The flushing of a nozzle usuallyoccurs at a data point of the flushing pattern that corresponds to adata point of the page of print data. The developed flushing pattern hasa pleasing appearance where the dots are arranged in an unstructuredpattern approximately matching the filter response in the human visualsystem.

In one embodiment, the flushing pattern is merged with print job datawith the print data occurs so that a data point of the pattern is notincluded where there exists a corresponding data point in the printdata. Because the merging of the flushing pattern with the print data isnot computationally intensive, there is no slow down of the throughputof printer.

FIG. 7 is a flow diagram illustrating one embodiment of merging aflushing layout with print job data. At processing block 705, the sheetdata is rasterized. At processing block 710, the sheet data ishalftoned. At processing block 715, a square inch block from the top ofthe sheet data is selected. At decision block 720, it is determined forevery point (i,j) of a page to be printed, whether print job dataexists. If job data exists at a point (i,j), flushing will be ignoredfrom the halftone sheet data at the position on the page where theflushing is set to occur.

However if the halftone sheet data does not include a data point at(i,j), the point (i,j) of the flushing pattern is combined into theprint job data, processing block 725. At processing block 730, thenumber of dots at every nozzle/column is calculated. At decision block735, it is determined whether the ink volume is sufficient. If thevolume is not sufficient, the next level of pattern (e.g., two dots orthree dots per inch) is combined to that nozzle/column until that nozzleejects minimum volume of ink at that frequency, processing block 740.Subsequently, control is returned to processing block 730. FIG. 8illustrates embodiments of flushing patterns.

If the volume is sufficient, the combined data is forwarded forprinting, processing block 745. This process prevents the flushing fromdirectly interfering with the integrity of the original print data, andwhere the flushing does occur, visually pleasing characteristics reducethe visibility of the flushing pattern, resulting in minimized impact onthe image integrity of the original print job. FIGS. 9A and 9Billustrates embodiments of merged flushing patterns with print job data.FIG. 9A illustrates a conventional flushing pattern in which theflushing dots are sprayed on data points, while in FIG. 9B, no dots areincluded at points (i,j) where print job data exists.

In another embodiment, multi-channel ink systems may also be flushed ina visually pleasing manner as explained below. For instance, manyprinters are capable of including more than one ink type, and all of theprint heads/nozzles using these different inks are flushed to preventdrying. This flushing could lead to noisy background on the paper, whichwill be objectionable for printer 130 users. According to oneembodiment, a visually pleasing flushing mask is generated as explainedabove, and the mask is de-interlaced and distributed among differentinks.

For example, printer 130 may have to flush six channels (e.g., Cyan(C),Magenta(M), Yellow(Y), Black(K), Reserve1(R1) and Reserve2(R2)).R1-channel and R2-channel can be any colorant inks. For the purpose ofthis description, assume R1-channel and R2-channel are also blackcolored inks. In one embodiment, the flushing pattern is de-interlacedinto 4 dot groups. Three of the dot groups will be used by R1-channel,R2-channel and the K-channel. The R1-channel and R2-channel could be onthe same engine or on a different engine but flushing on the same sideof the paper. The fourth group will be used by C, M, Y channels suchthat they form a composite black drop when printed. This will make thebackground appearance as single black colored ink flushing patterninstead of a noisier and denser pattern that present system produces.FIG. 10 illustrates one embodiment for flushing a multi-channel inksystem.

Embodiments of the invention may include various steps as set forthabove. The steps may be embodied in machine-executable instructions. Theinstructions can be used to cause a general-purpose or special-purposeprocessor to perform certain steps. Alternatively, these steps may beperformed by specific hardware components that contain hardwired logicfor performing the steps, or by any combination of programmed computercomponents and custom hardware components.

Elements of the present invention may also be provided as amachine-readable medium for storing the machine-executable instructions.The machine-readable medium may include, but is not limited to, floppydiskettes, optical disks, CD-ROMs, and magneto-optical disks, ROMs,RAMs, EPROMs, EEPROMs, magnetic or optical cards, propagation media orother type of media/machine-readable medium suitable for storingelectronic instructions. For example, the present invention may bedownloaded as a computer program which may be transferred from a remotecomputer (e.g., a server) to a requesting computer (e.g., a client) byway of data signals embodied in a carrier wave or other propagationmedium via a communication link (e.g., a modem or network connection).

Throughout the foregoing description, for the purposes of explanation,numerous specific details were set forth in order to provide a thoroughunderstanding of the invention. It will be apparent, however, to oneskilled in the art that the invention may be practiced without some ofthese specific details. Accordingly, the scope and spirit of theinvention should be judged in terms of the claims which follow.

What is claimed:
 1. A method comprising: receiving print job data;rasterizing the print job data to generate image data for each page ofthe print job data to be printed; calculating a coverage per unit areafor two or more color planes of each page of the image data; calculatinga flushing mask to flush nozzles of the inkjet print head; and whereincalculating a coverage per unit area comprises: generating a coveragefunction; and converting the coverage function to provide an estimate ofa probability function.
 2. The method of claim 1 wherein the flushingmask is calculated to achieve one or more criteria.
 3. The method ofclaim 2 wherein the one or more criteria comprise at least one ofminimizing an amount of flushing ink, achieving a pleasing human visualsystem (HVS) mask for each of the two or more color planes and adjustingflushing density based on digital counts for the two or more colorplanes.
 4. The method of claim 1 wherein converting the coveragefunction comprises: normalizing the coverage function by dividing thecoverage function by 100; and inverting the normalized coveragefunction.
 5. The method of claim 1 further comprising re-scaling toaccount for the difference between the coverage function above and theeffect of nozzle clogging not related to ink volume ejected.
 6. Aprinter comprising: a control unit to receive print job data, rasterizethe print job data to generate image data for each page of the print jobdata to be printed, calculate a coverage per unit area for two or morecolor planes of each page of the image data and calculate a flushingmask; and an inkjet print head having a plurality of ink nozzles flushedaccording to the flushing mask; wherein calculating a coverage per unitarea comprises the control unit generating a coverage function andconverting the coverage function to provide an estimate of a probabilityfunction.
 7. The printer of claim 6 wherein converting the coveragefunction comprises the control unit normalizing the coverage function bydividing the coverage function by 100 and inverting the normalizedcoverage function.
 8. An article of manufacture comprising amachine-readable medium including data that, when accessed by a machine,causes the machine to perform operations comprising: receiving print jobdata; rasterizing the print job data to generate image data for eachpage of the print job data to be printed; calculating a coverage perunit area for two or more color planes of each page of the image data;calculating a flushing mask to flush nozzles of the inkjet print head;and wherein calculating a coverage per unit area comprises: generating acoverage function; and converting the coverage function to provide anestimate of a probability function.
 9. The article of manufacture ofclaim 8 wherein converting the coverage function comprises: normalizingthe coverage function by dividing the coverage function by 100; andinverting the normalized coverage function.